The present invention relates to mechanical devices used for making secure electrical connections to superconductive coils disposed within cryostats. More particularly the present invention relates to exteriorly operated electrical connection mechanisms for use in superconductive magnets used in nuclear magnetic resonance (NMR) diagnostic medical imaging.
In the generation of tomographic and other images and data in medical NMR diagnostic systems, it is necessary to provide a highly uniform, high strength magnetic field. This field may be provided by permanent magnets, by conventional resistive magnets, or, by superconductive magnet structures. It is these latter superconductive structures which are of concern in the present invention. In these systems, main superconductive electrical coil windings are disposed in a vessel containing a cryogenic fluid such as liquid helium. The superconductive winding material is immersed in such a fluid so as to keep its temperature sufficiently low that the superconductive state is maintained. In order to provide the desired magnetic field uniformity, correction coils, similarly cryogenically disposed, are also employed. The main coils and the correction coils are disposed within a cryostat which is essentially a thermal insulation device. The superconductive windings exhibit the particular advantage that electrical energy need not be supplied to the circuit once the main coils and the correction coils are properly energized. However, in general, electrical connection must be made to these interior coil windings at various intervals. For example, in the case of a sudden magnetic quench condition in which the superconductive windings undergo a transition to the normal, resistive state, it is necessary to reconnect and re-energize the coils. Additionally, it is desirable to be able to adjust the currents in the correction coils from time to time to compensate for changes in the uniformity of the magnetic field as a result of changes in the position of external ferromagnetic objects. In a typical magnet, the main magnet coils typically carry a current of approximately 1,000 amperes while the correction coil currents are typically no more than approximately 50 amperes.
While the correction coils and the main magnet coils typically comprise superconductive material, circuit energization is generally accomplished by means of normal (that is, resistive) conductors which penetrate the nested set of cryostat vessels without significantly impairing their insulating function or increasing the rate of helium evaporation. Since it is desirable to make electrical connections to the interior of the cryostat retractable in the sense that they can be removed from the cryostat vessel, several significant criteria must be met. Firstly, it must be noted that, because of the extremely low temperatures at which the electrical contact is made, there is a very strong tendency for frost to form on the electrical contacts. This frost typically includes both ice and a frost of solidified air itself. This frost, whether ice, air or both, can significantly impede the formation of a good electrical connection between the interior cryostat circuit and an exterior energizing source. Additionally, it is also significant to note that the electrical and mechanical properties of the contact surfaces must be adequate at the cryogenic temperatures employed. In particular, the hardness and electrical conductivities of the contacts employed must be such that good electrical contact is made and maintained even at low temperatures. Satisfactory electrical contact is important because a high resistivity contact junction will result in the generation of heat in accordance with the formula I.sup.2 R, where I is the current and R is the contact electrical resistance. Again, it should be borne in mind that currents of approximately 1,000 and 50 amperes typically flow in such circuits so that even slight increases in electrical resistance can produce high levels of thermal energy which deleteriously act to unnecessarily boil off cryogenic coolants such as liquid helium which is relatively expensive. Thirdly, since the lead is desired to be retractable, it is necessary to provide a mechanism in which excessive wear, erosion and abrasion of the contact surfaces does not occur. This latter criteria helps to insure consistency of device performance and magnetic field strength uniformity. In sum, electrical contacts that are normally used at ambient temperature conditions are not suitable for low temperature applications because the hardness of the mating surfaces is increased and frost accumulation will develop on the contact surfaces. Consequently, electrical contacts which exhibit a low resistance at ambient temperatures have considerably higher resistance at cryogenic temperatures. This is of particular concern in those situations in which high current levels can produce ohmic heating at the contact surface and can therefore result in excessive cryogen consumption. All of these factors are complicated by the fact that the contact surface typically lies deep within the interior of the cryostat structure and by the fact that material properties such as hardness and resistivity are highly temperature dependent. Accordingly, it is seen that an electrical contact mechanism and lead assembly is required to overcome these problems.